Mobile telephones and similar communication devices are becoming more popular, and are being used for an ever-increasing number of services, or functions. These multiplying services impose greater demands on mobile communication networks, and require ever-more efficient methods of transmission over the air interface.
Mobile telephones are in essence portable radios, used by subscribers to communicate with a mobile telecommunication network. In order to give mobile subscribers the capability of connecting when they travel, numerous antennas are placed throughout the network coverage area and generally speaking, a mobile station communicates with the network through the antenna nearest or best-suited to its current location.
FIG. 1 is a simplified block diagram illustrating selected components of a mobile telecommunications system 100, such as one in which the present invention may be implemented. Mobile telephone 101 is currently within range of antenna 110 and able to establish a communication channel using appropriate network protocols. Transmissions from mobile station 101 received at antenna 110 are processed by base station 130 operating under the direction of base station controller 140. Although only one mobile station 101 is shown, in an actual mobile network there could be a great many, several of which are communicating via antenna 110 and base station 130.
Base station 130, as already mentioned, is itself under the control of base station controller 140. Base station controller 140, in turn, is connected with mobile switching center 150. Mobile switching center 150 handles communications originating with mobile station 101 and routes them through mobile telecommunication network 100. Also shown in FIG. 1 are, antennas 105 and 115, which corresponding with base stations 125 and 135, respectively. Base stations 125 and 135 are both controlled by base station controller 145, which also routes communications through mobile switching center 150. Mobile switching center 150 is one example of a number of such switches (not shown but) normally present in a mobile telecommunication network for routing calls from one subscriber to another or to other communication networks when appropriate. Calls to and from other networks will usually be routed through a gateway having an inter-working function (also not shown) that not only transfers information from one network to another but also reformats it as necessary to adjust to varying protocols. Other well-known communication networks, for example, include the public-switched telephone network (PSTN) and the Internet.
Returning to FIG. 1, the area covered by an antenna is often referred to as a cell. The network 100 coverage area is divided into a large number of such cells, three of which are delineated with broken lines. Although cells are often thought of as separate areas, they may overlap with each other or even be completely enclosed within other cells. Where cell coverage overlaps, the communicating base station may be the one best able to handle the additional traffic, or the choice may be made for other reasons. A mobile station traveling from one cell to another switches from communicating through one antenna to communicating through another in a process called handover. During the handover process, the mobile station may actually be communicating with or through two or more antennas.
The cellular architecture of mobile telecommunication network 100 means that mobile stations may use relatively low power for transmitting to a nearby base station. This of course means less battery capacity is required, an important consideration for a portable device. It also means that radio signals transmitted by a mobile station (as well as those of the base station with which it is communicating) will generally not interfere with those of mobile stations in non-adjacent cells. Communication frequencies may therefore be “re-used” in distant cells, vastly increasing the traffic capacity of telecommunication network 100.
Within a given cell, multiple mobile stations may engage in concurrent communication sessions when one of several multiple access techniques is employed. In frequency division multiple access (FDMA), different portions of the network operator's allocated frequency spectrum are used to create different channels, with each mobile station is assigned one or more frequencies as needed. These assignations are normally for one communication session only, and when a call is complete the frequencies are released for use by another. In time division multiple access (TDMA), each frequency is further divided into a number of time slots, creating even more channels. A TDMA network may in this way assign multiple mobile stations to a single frequency, with each transmitting only in their assigned time slot (or slots). The slots are long enough and occur with sufficient regularity that the discontinuous nature of even a voice transmission is generally imperceptible to subscribers.
Rather than assigning frequency channels, code division multiple access (CDMA) assigns a unique spreading code or codes to each mobile-station communication session. Signals are spread using the spreading code prior to transmission such that the receiver, knowing the spreading-code assignment, decodes only those transmissions it is intended to receive. The spreading codes are generated to be mutually orthogonal so that they do not interfere with each other.
These multiple access techniques have permitted rapid growth in the number of mobile telephone subscribers. For most, an acceptable communication channel is almost always available except perhaps during peak periods in high-traffic areas. At the same time, advances in technology have made mobile stations both smaller and less expensive. In other words, the cost of mobile station use in terms of both inconvenience and money has gone down. As performance and affordability continue to improve, many subscribers are beginning to use their mobile stations instead of, rather than in addition to their familiar wire line telephones.
The different uses in which mobile stations may be employed are also increasing. Originally (and often still primarily) used for voice communication, they frequently communicate other types of information as well. For example, a short message service (SMS) message may be sent from or received by a mobile station subscriber. These simple text messages permit brief communications between parties even if they are not simultaneously available. Longer text messaging is now often available as well, in some cases being achieved by concatenating a series of SMS messages.
Email applications are also becoming more common, permitting wireless mobile station users to send and receive email using devices that may be equipped with a full keyboard for composition. Access to the Internet creates still more uses for mobile stations. If network 100 provides an appropriate gateway (not shown), subscribers may request a Web page (or perhaps an abbreviated version of one from a Web site. In some applications, a personal computer may actually use the communication capability of a wireless mobile station for full Internet access.
As should be apparent, the amount of information, both voice and data, being transmitted over mobile telecommunication networks is much greater now than was initially experienced, and is still growing rapidly. This means that a tremendous amount of data must be handled by the network. It should be remembered, however, that most, if not all, of the communication within telecommunication network 100 takes place over a wire, cable, optical fiber or some other form of relatively high-speed connection, except for the radio connection between mobile station and base station. This radio channel, often called the air interface, is therefore the most resource-critical part of the entire communication path. Efficiency gains realized in the air interface typically affect system capacity and performance.
Needed therefore is a modified transmission scheme that more efficiently utilizes bandwidth and power resources without sacrificing quality of service. The present invention provides just such a solution.